1. Field of the Invention
This invention relates generally to measurement and data acquisition systems and, more particularly, to a flexible converter interface for use in analog-to-digital and digital-to-analog systems.
2. Description of the Related Art
Scientists and engineers often use measurement systems to perform a variety of functions, including measurement of a physical phenomena or unit under test (UUT), test and analysis of physical phenomena, process monitoring and control, control of mechanical or electrical machinery, data logging, laboratory research, and analytical chemistry, to name a few examples.
A typical measurement system comprises a computer system with a measurement device or measurement hardware. The measurement device may be a computer-based instrument, a data acquisition device or board, a programmable logic device (PLD), an actuator, or other type of device for acquiring or generating data. The measurement device may be a card or board plugged into one of the I/O slots of the computer system, or a card or board plugged into a chassis, or an external device. For example, in a common measurement system configuration, the measurement hardware is coupled to the computer system through a PCI bus, PXI (PCI extensions for Instrumentation) bus, a GPIB (General-Purpose Interface Bus), a VXI (VME extensions for Instrumentation) bus, a serial port, parallel port, or Ethernet port of the computer system. Optionally, the measurement system includes signal conditioning devices which receive field signals and condition the signals to be acquired.
A measurement system may typically include transducers, sensors, or other detecting means for providing “field” electrical signals representing a process, physical phenomena, equipment being monitored or measured, etc. The field signals are provided to the measurement hardware. In addition, a measurement system may also typically include actuators for generating output signals for stimulating a unit under test.
Measurement systems, which may also be generally referred to as data acquisition systems, may include the process of converting a physical phenomenon (such as temperature or pressure) into an electrical signal and measuring the signal in order to extract information. PC-based measurement and data acquisition (DAQ) systems and plug-in boards are used in a wide range of applications in the laboratory, in the field, and on the manufacturing plant floor, among others.
In a measurement or data acquisition process, analog signals may be received by a digitizer, which may reside in a DAQ device or instrumentation device. The analog signals may be received from a sensor, converted to digital data (possibly after being conditioned) by an analog-to-digital converter (ADC), and transmitted to a computer system for storage and/or analysis. When a measurement system generates an output analog signal, the computer system may generate digital signals that are provided to one or more digital-to-analog converters (DACs) in the DAQ device. The DACs may convert the digital signal to an output analog signal that is used, e.g., to stimulate a UUT.
A DAQ device typically includes custom circuitry to interface with specific ADCs and DACs. For example, a custom converter interface may be configured to interface with a serial ADC. In another example, a custom converter interface may be configured to interface with a parallel ADC. In general, an ADC may return data in a particular mode, either binary or two's complement. Depending on the type of data (i.e., unipolar or bipolar) being sampled by the ADC, data conversion may be necessary. The conversion from binary to two's complement and vice versa may be accomplished by inversion of the most significant bit (MSB).
Data conversion may be necessary in order to return data to software in the correct format. When sampling a unipolar signal, software typically interprets the data returned in straight binary mode. When sampling a bipolar signal, software usually interprets data in two's complement mode. The primary reason for software interpreting data in this manner is software efficiency and it is particularly important when the data storage type of the data is the same width as the resolution of the data from the converter. If the data returned is a unipolar value, all values will typically be positive. However, if two's complement representation of this data is used, one bit usually represents the sign and the remaining bits represent the data. For 16-bit unipolar data in a 16-bit integer format, one bit of resolution may need to be sacrificed in order to represent the sign. Since it is known that unipolar data is always positive, straight binary representation (which does not use a sign bit) is typically a more efficient way of interpreting the data. When bipolar data is returned, the sign bit is typically part of the data and therefore two's complement representation may be used without sacrificing any resolution.
In some custom ADC interfaces, programmable inversion of the MSB (to accomplish the data conversion) may be performed by tying the MSB of the data received from the ADC as well as a signal that specifies the type of data, i.e., unipolar or bipolar, to a logic unit that decides whether or not to invert the MSB. The output of this logic unit may be two signals: a MSB signal and a sign extension signal. Sign extension may be accomplished by tying the sign extension signal to all the upper bits of the data path. In the case of a 12-bit ADC with a 16-bit data path, the sign extension signal may be tied to lines 12 through 15 of the 16-bit data path. This setup is typically difficult to realize because the board designer may need to correctly identify and feed the MSB of the ADC into the logic, as well as tie the right lines of the FIFO to the sign extension signal and the MSB output signal. Also, this logic setup typically assumes that the ADC returns binary data. If the ADC returned two's complement data, an inverter may be placed on the MSB of the data coming into the logic unit. However, both binary and two's complement ADCs may not be supported on the same implementation without changing the design of the ADC interface. Also, in another implementation, to support a 24-bit ADC, the ADC interface may require twenty-four dedicated analog input (AI) FIFO pins. This may allow the designer to perform sign extension by tying all the upper bits to the sign extension signal; however, at least twenty-four dedicated pins may be required. I/O on logic chips is typically very expensive. Therefore, it may be necessary to keep the I/O pin count as low as possible to keep the price of the device low. In some cases, functionality may be reduced to reduce the pin count.
Data acquisition converter interfaces (e.g., ADC and DAC converter interfaces) have primarily been designed with custom interfaces. FIFO widths are typically chosen in accordance with the resolution of the converter. Sign extension for analog input usually requires custom schematic work. Interfaces to serial converters (e.g., serial ADC converters) are typically developed around the communications protocol for the corresponding device. However, custom circuitry typically means less leverage of design, longer design cycles for new products, more expensive product costs, and lower yields.